A COMPREHENSIVE REVIEW ON THE USE OF MICROBES IN MINERAL RECOVERY AND ACID MINES

Biomining is successful on the commercial scale for the recovery of various metals such as copper and ores from their ores. The methodology involved in this is primarily chemistry-driven and is a combination of ferric and hydrogen ions which varies on the type involved. Hydrogen ions present here are produced by the activity of chemolithotrophic bacteria growing in a highly acidic environment. Bioleaching reactions, on the other hand, the role of microorganisms involved, and whether the reaction carried out are direct or indirect are discussed below. In places where the availability of oxidants to sulfide mineral surfaces is exposed due to mining, the acid mine drainage tends to contaminate the surroundings as well. Microorganisms that mainly consist of autotrophic and heterotrophic archaea and bacteria, take part in catalyzing iron and sulfur oxidation which determines the release of metals and sulfur into the surrounding. Indication towards the physiological synergy in sulfur, iron, and carbon flow in the microbial community is assessed. With this, the development, and future aspects of this with the challenges in mind are described as well.

From Laboratory towards Industrial Operation: Biomarkers for Acidophilic Metabolic Activity in Bioleaching Systems

In the actual mining scenario, copper bioleaching, mainly raw mined material known as run‐of‐mine (ROM) copper bioleaching, is the best alternative for the treatment of marginal resources that are not currently considered part of the profitable reserves because of the cost associated with leading technologies in copper extraction. It is foreseen that bioleaching will play acomplementary role in either concentration—as it does in Minera Escondida Ltd. (MEL)—or chloride main leaching plants. In that way, it will be possible to maximize mines with installed solvent‐extraction and electrowinning capacities that have not been operative since the depletion of their oxide ores. One of the main obstacles for widening bioleaching technology applications is thelack of knowledge about the key events and the attributes of the technology’s critical events at the industrial level and mainly in ROM copper bioleaching industrial operations. It is relevant to assess the bed environment where the bacteria–mineral interaction occurs to learn about the limiting factors determining the leaching rate. Thus, due to inability to accurately determine in‐situ key variables, their indirect assessment was evaluated by quantifying microbial metabolic‐associated responses. Several candidate marker genes were selected to represent the predominant components of the microbial community inhabiting the industrial heap and the metabolisms involved in microbial responses to changes in the heap environment that affect the process performance. The microbial community’s predominant components were Acidithiobacillus ferrooxidans, At. thiooxidans, Leptospirillum ferriphilum, and Sulfobacillus sp. Oxygen reduction, CO2 and N2 fixation/uptake, iron and sulfur oxidation, and response to osmotic stress were the metabolisms selected regarding research results previously reported in the system. After that, qPCR primers for each candidate gene were designed and validated. The expression profile of the selected genes vs. environmental key variables in pure cultures, column‐leaching tests, and the industrial bioleaching heap was defined. We presented the results obtained from the industrial validation of the marker genes selected for assessing CO2 and N2 availability, osmotic stress response, as well as ferrous iron and sulfur oxidation activity in the bioleaching heap process of MEL. We demonstrated that molecular markers are useful for assessing limiting factors like nutrients and air supply, and the impact of the quality of recycled solutions. We also learned about the attributes of variables like CO2, ammonium, and sulfate levels that affect the industrial ROM‐scale operation.

Dissolution and Passivation of Chalcopyrite during Bioleaching by Acidithiobacillus ferrivorans at Low Temperature

Our knowledge on the dissolution and passivation mechanisms of chalcopyrite during bioleaching at low temperature has been limited to date. In this study, an Acidithiobacillus ferrivorans strain with high tolerance to heavy metals and UV radiation was used for chalcopyrite bioleaching. At 6 °C, no apparent precipitate was detected on the mineral surface after bioleaching using a scanning electron microscope (SEM). X-ray diffraction (XRD) revealed that the ore residue contained only chalcopyrite and quartz. X-ray photoelectron spectroscopy (XPS) analysis revealed that the content of S0 on the mineral surface remained low and the ratio of SO42− decreased from 46.7% to 20.9%, but the amount of Sn2− increased from 10.4% to 21.4% after bioleaching. Expression of five critical iron- and sulfur-oxidation genes during bioleaching was analyzed using quantitative real-time PCR. The gene rusA had higher expression in the mid-log phase than in the stationary phase but hdrA and cyoC1 showed an opposite trend. All genes had higher expression at 6 °C than at 28 °C, so as to compensate for the decline in the enzyme activities. The study revealed that polysulfide was the most plausible passivating substance at 6 °C, and the strain can maintain the iron- and sulfur-oxidation activities during low-temperature bioleaching.

Bioleaching of Chalcopyrite by Thermophilic Archaea

Bioleaching and biooxidation of sulfidicores and concentrates generate very high acidities and a great of heat, which rise the temperature in the reactors or heaps, and accumulate the sulfur on the surface of the ores. Extremely thermoacidophilic archaea, mainly from the genus ofAcidianus, Sulfolobus,Metallosphaeraandsulfurisphaera, have great potential to contribute to biomining processes for their inherent tolerance for low pH, high temperature, and high-soluble metal concentrations. Species of the genusMetallosphaeratypically grow by aerobic respiration on CO2with S0, tetrathionate (S4O62+), and Fe2+as electron donors, particularly suitble for metal extraction under high temperature by their iron- and sulfur-oxidation ability. Several species fromMetallosphaeraandAcidianusgenerawere investigated for their ability and conditions to dissolve various ores under a range of conditions. All of them showed good performance in copper extraction from chalcopyrite, with strainM.cuprinaAr-4 displaying higher activity than others. Surface analysis of chalcopyrite leached with the strain showed the leaching products accumulated on the ores. Our study will cover new understandings on the application of these thermoacidophilic archaea in biomining.

How the RegBA Redox Responding System Controls Iron and Sulfur Oxidation in Acidithiobacillus ferrooxidans

Valuable metals as well as ferrous iron and sulfur compounds are released from ore by ferric iron and sulfuric acid chemical attack. Biomining microorganisms allow the recycling of these products by oxidizing ferrous iron and/or sulfur compounds. The energy released from the oxidation of these substrates is used for the growth of the acidophilic chemolithoautotrophic bacterium Acidithiobacillus ferrooxidans. The respiratory pathways involved in these respiratory processes have been deciphered and the expression of the genes encoding these redox proteins is dependent on the electron donor present in the medium. Furthermore, in the presence of both ferrous iron and sulfur, the genes involved in iron oxidation are expressed before those involved in sulfur oxidation. We propose that the global redox responding two component system RegBA is responsible for this regulation since (i) the redox potential increases during iron oxidation but remains stable during sulfur oxidation and (ii) the transcriptional regulator RegA binds the regulatory region of a number of genes/operons involved in iron and sulfur oxidation. To understand the mechanism of the At. ferrooxidans RegBA system, the regA gene and the DNA corresponding to the DNA binding domain of RegA were cloned in an expression plasmid in Escherichia coli. The recombinant proteins, RegA and RegA-HTH respectively, were purified. The binding of RegA-HTH, phosphorylated and unphosphorylated RegA on the regulatory region of some target operons have been compared by gel shift mobility assay.

Characterization of Oxidizing Activity of a Microbial Community in an Industrial Bioleaching Heap

In order to explore new options to optimize the low-grade copper ore bioleaching process, it is important to understand the kinetics of microbial oxidation at industrial level. This work studies the changes of iron and sulfur oxidation rates of microbial communities in solution from an industrial low grade copper bioleaching heap process at Escondida Mine in Chile. Pregnant leach solution (PLS) samples were analyzed periodically to determine physico-chemical parameters. The total numbers of the different microorganism species in industrial samples were determined by Real Time PCR. In addition, Most Probable Number assays (MPN) were performed for iron and sulfur oxidizing microorganisms. Kinetics incubation tests of PLS in the presence of iron or sulfur were performed to study the iron and sulfur oxidation, in total, 102 oxidation profile tests were obtained. Based on the oxidation profiles obtained, the tests were divided into four groups, labeled as fast, normal, stepped shape, and incomplete. The grouping system was established by considering oxidation time and rates, during the initial oxidation stages and accounted for any lag phase. A data mining technique, called decision trees was used to analyze the data and to generate rules that represented patterns in the data. Strong correlations were found between the predominant microorganisms and the behavior of the oxidation tests. Preliminary results indicate that the magnitude order of MPN of the iron oxidizing microorganisms is an important factor in the microbial oxidizing activity, followed by the predominant specie within the microbial population, PLS temperature and Eh.

Regulation of the Iron and Sulfur Oxidation Pathways in the Acidophilic Acidithiobacillus Ferrooxidans

The acidophilic and strictly chemolithoautotrophic bacterium Acidithiobacillus ferrooxidans oxidizes ferrous (Fe(II)) to ferric (Fe(III)) iron and reduced inorganic sulfur compounds (RISC) to sulfuric acid, in oxic conditions. The redox proteins involved in the electron transfer between Fe(II) and oxygen are encoded in the same transcriptional unit, the rus operon. The expression of this operon is induced in the presence of Fe(II), but not Fe(III), and is not repressed in the presence of sulfur (S0). A number of genes differentially expressed in iron or sulfur conditions have been identified by microarrays transcript profiling. We show here that the presence of Fe(II) induced the expression of the genes involved in iron oxidation and repressed the expression of the genes involved in RISC oxidation. Identification of the regulator(s) involved in this transcriptional regulation is underway. Two genes encoding putative regulators belonging to two transcriptional units located downstream from the rus operon have been cloned. One regulator with a putative ironsulfur cluster belongs to the IscR family and the other belongs to the two component sensor/regulator family. Expression of both genes is induced in the presence of Fe(II) and is not repressed by S0. The recombinant proteins have been purified and gel shift assays with the target regulatory regions are in progress.